1. Field of the Invention
The present invention relates generally to safety equipment for protecting the head of a human. More particularly, the present invention relates to protective headgear for reducing the probability of head injury and mitigating the effects of impacts to the area of the cranium.
2. Related Art
Protective headgear, in various forms, has been known for millennia. More recently, headgear suited for cranial protection in outdoor recreational activities, including biking, climbing, skydiving, skateboarding, rollerblading, skiing, snowboarding, and the like, have been developed. These typically include a shell configured to be impact resistant, and a liner, typically formed of a energy absorbing, and/or shock mitigating material, such as a foamed polymeric resin, and a retention system. The retention system typically comprises one or more straps which attach to the shell and extend down around at least a chin portion of the head of a wearer. A clasp, buckle or snap of some kind, or other means for releasably fastening the strap(s) so as to retain the headgear on the head of the wearer is usually provided.
The shell is typically formed of an impact resistant and relatively hard material. This is to mitigate impacts by resisting penetration and spreading resistance to the impact force laterally from an impact point. Among the advantages this provides is enabling more of the liner to be brought into play in mitigating the impact, rather than relying on the portion of the liner directly below a point of impact.
The liner typically is a closed cell foam or a combination of open and closed cell foams. Some liner systems are designed to convert impact energy to heat through deformation beyond the elastic limit of the material(s). Expanded polystyrene and other relatively rigid but progressively collapsible foamed resins are examples of such systems. Other systems are designed to shed energy by conversion to heat by deformation within the elastic range of the material(s). A combination of deformation within and without the elastic range is known. For example, some systems have a deformable layer adjacent the shell, and an elastomeric layer between the head of the wearer and the deformable layer. This inner layer is often pads of an open cell foam (or a closed cell foam) adhesively attached to the deformable layer. The elastomeric layer also allows at least a small amount of adaptability to differing head sizes. Over-compression of the elastomeric foam can reduce its effectiveness somewhat, and a too-loose fitting helmet can shift and expose a portion of the head sought to be protected, so such liners, and shells, are sized for different sizes of heads, and only a limited amount of variation is accommodated.
Typically the liner also serves, along with the shell, to spread impact forces laterally, to reduce the force per unit area on the head of the wearer. So at least some rigidity, or more properly shear force transfer, is desirable. But this must be balanced with the energy absorption properties to achieve good results. One approach taken to providing for this dual role of the liner is to provide a composite liner of differing material layers. An example of such a system has just been mentioned. At least one layer that has a higher shore hardness for spreading of the impact force, and at least one layer of lower shore hardness for absorption, or “cushioning” have been used. Liners having a multiplicity of layers are known. U.S. Pat. No. 6,425,141 sets forth an example of such a system.
The protective headgear can vary somewhat depending on the use to which it will be put. For example, protective helmets purpose designed for bicycle racing tend to be more aerodynamically shaped and ventilated than those designed with rock climbing or spelunking in mind. However, many helmet designs have typically been used in more than one activity. That is to say, there is a perception among some, that a helmet is a helmet, and the important thing is that a participant in an activity wear a helmet, not the particulars of the helmets design.
While it is usually true than any helmet is better than no helmet, nevertheless, some factors that may affect helmet performance in one activity may not obtain in another, and so if a helmet is designed with the former in mind, the helmet may not be totally adequate in the latter. For example, in whitewater sporting activities hydrodynamic forces can be very strong. The flow of water can shift a helmet on the head of the wearer, for example rotating it up and back, exposing the forehead area of the cranial portion of the wearers head intended to be protected. Such forces are not so important in skateboarding, hockey, rock climbing, etc. and are not typically a major concern in the design of helmets for such activities.
However, a wearer of a helmet designed with skateboarding or rock climbing in mind may be at increased risk of injury or death due to shifting of the helmet if the helmet is worn for head protection during kayaking or skydiving activities. For example, hydrodynamic or aerodynamic forces can shift such a helmet as discussed above.
In the whitewater-sporting activity example, a non-fatal head injury can result in death due to secondary causes such as drowning or blunt-force trauma to other parts of the body which arise because the non-fatal head injury caused temporary unconsciousness and loss of breathing control and the ability to avoid hazards. Therefore, in this activity, protection of the head is if anything only more important because any head injury resulting in unconsciousness is potentially fatal due to secondary causes such as those mentioned.